I saw that Steve Kim mentioned this on Twitter, so I made a quick write up on how to do this. I got two years of PC Game Pass doing this for the effort of ten minutes.
- Open the Game Pass app on Desktop. Go to Settings, and under Manage, click "Xbox Game Pass PC Subscription."
- Click "Extend" and "Manage" - optional: put them in separate windows side by side instead of the two tabs that they open in to do this faster
- Click "Extend" on the extend tab/window you opened and accept the checkout with whatever payment you have. Takes a few seconds.
- If you do not have the Extend button, skip to the next step and you'll get it to appear once you go through the process.
- Refresh the Manage tab/window that you opened while the payment is going and you'll see "Cancel Subscription," possibly before the checkout finishes. Click this.
- It'll ask if you want to turn off reoccurring billing. Click yes.
- Refresh the left side and, like before, it might show up before the reoccurring billing process is over.
- Repeat 3-6.